Multiple sclerosis (MS) is a chronic inflammatory neurodegenerative disorder of the central nervous system (CNS) and is the leading cause of disability in young adults, afflicting some 400,000 U.S. citizens and generating an economic burden of approximately $10 billion annually. MS results from an incompletely understood interaction between genetic and environmental factors that triggers an autoimmune response against CNS myelin. Chronic CNS inflammation induces pro-inflammatory programs in CNS-resident cells such as astrocytes and microglia, which are not responsive to the therapeutic approaches currently available for MS. Astrocytes are abundant CNS-resident cells which participate in multiple aspects of CNS homeostasis in health and disease, including pro-inflammatory signaling in the context of MS and its animal model, experimental autoimmune encephalomyelitis (EAE). Thus, the study of the mechanisms that regulate astrocyte pro-inflammatory activities may identify mechanisms of disease pathogenesis in MS, as well as novel efficacious therapies, particularly for its progressive phase. In previous studies focused on environmental factors in MS, we identified a signaling pathway in astrocytes that is controlled by environmental pollutants, and drives astrocyte pathogenic activities that promote inflammation and neurodegeneration in EAE and MS. Specifically, we found that the endoplasmic reticulum (ER)-localized receptor SigmaR1 stabilizes the inositol requiring enzyme 1-alpha (IRE1a), leading to the activation of the transcription factor X-box binding protein 1 (XBP1) which promotes pro-inflammatory gene expression in astrocytes. In genetic perturbation studies we demonstrated that SigmaR1-driven IRE1a-XBP1 activation boosts the expression of pro-inflammatory and neurotoxic transcriptional programs in astrocytes such as Nos2, Ccl2, Il6, Csf2 (GM-CSF), and Csf2ra (the GM-CSF receptor) during EAE. Moreover, we detected increased IRE1a-XBP1 activation in astrocytes localized to MS lesions. I hypothesize that SigmaR1-IRE1a- XBP1 signaling drives astrocyte pathogenic activities in EAE and MS. Thus, I propose the following Aims: AIM 1: Mentored phase (K99). Define astrocyte subpopulations driven by SigmaR1-IRE1a-XBP1 signaling (XBP1+ astrocytes) in both EAE (Aim 1.1) and MS (Aim 1.2) using single-cell RNA sequencing (scRNA-seq). AIM 2: Mentored phase (K99). Test the therapeutic potential of suppressing XBP1 signaling with clinically- relevant SigmaR1 inhibitors using EAE mouse models (Aims 2.1-2.2), and scRNA-seq (Aim 2.3). AIM 3: Independent investigator phase (R00). Study the regulation of GM-CSF signaling in XBP1+ astrocytes using spatial transcriptomic approaches including NICHE-seq (Aim 3.1) and MERFISH (Aim 3.2). Taken together, these studies will define a novel disease-associated astrocyte population, identify the molecular mechanisms that control it, and evaluate the therapeutic value of its pharmacologic manipulation.